Two familiar transmission media for high frequency electromagnetic energy are wave guides and microstrip circuits. Waveguides are hollow conductive conduits generally having a circular or rectangular cross section and are appropriate where transmission of energy from point to point with very low loss is desired. Microstrip circuits consist of a ground plane and a signal carrying microstrip separated by a dielectric material. Microstrip circuits are more subject to radiation and other losses than are waveguides, but may be inexpensively constructed by familiar photo etching techniques. Furthermore, signal processing components and microstrip interconnections are easily integrated onto a single dielectric substrate requiring less space than an equivalent waveguide circuit. In some systems, such as radar systems, it is necessary to utilize both microstrip and waveguide transmission media in different portions of the system. This, of course, requires the use of microstrip to waveguide transition apparatus which efficiently couples energy propagating in the one medium to the other medium.
It has been standard practice in the art to achieve microstrip to waveguide transitions through end-launch techniques. Several examples of such techniques are discussed in U.S. Pat. No. 2,825,876 for Radio Frequency Transducers, issued Mar. 4, 1958 to D. J. Le Vine et al. The salient feature of end-launch transitions is that the direction of propagation in the waveguide is parallel to that in the microstrip. In a system requiring different directions of propagation some form of waveguide apparatus must be utilized to change that direction. Various waveguide components such as Tees or corners are well-known for accomplishing such a change of direction, but they require substantial space and are costly in comparison with microstrip circuits. The size and weight represented by a waveguide Tee or corner are vital factors if the system is to be a part of an airborne vehicle or other compact, lightweight device. For instance, a guidance radar for use in a small missile may have no extra space or payload margin for bulky waveguide components.
U.S. Pat. No. 3,579,149 for Waveguide To Stripline Transition Means, issued May 18, 1971 to Kurt G. Ramsey discloses a right angle transition involving a waveguide and a stripline circuit, which is somewhat similar to a microstrip circuit. This transition, however, utilizes a waveguide Tee to change the direction of propagation and the plane of the E-field prior to coupling to the stripline circuit, thus entailing almost the same bulk as an end-launch transition and a subsequent Tee or corner.